1. Field of the Invention
The present invention relates to apparatus for indicating the state of a telephone line by indication means, and more particularly the present invention is directed to means for effecting power saving in such indicator apparatus.
2. Description of the Prior Art
In the art there is well known a full-duplex data transmitting and receiving system on a telephone line employing acoustic couplers provided in terminal apparatus at both ends of the telephone line. FIG. 1 is a schematic block diagram useful in understanding the common procedure of data transmission and reception in such a full-duplex system.
In FIG. 1, reference numerals 1 and 4 designate terminal equipment and numerals 2 and 3 denote acoustic couplers used for transmitting and receiving data. When some data has to be transmitted and received through the shown system, at first the calling subscriber originates a call directed to the called station by means of its telephone station and makes an arrangement by a voice communication. Then, the calling subscriber sets his or her handset on the acoustic coupler 2 and sets the calling-called switch to its "calling" position on the operation console of the coupler. On the other side, the called subscriber sets his or her handset on the acoustic coupler 3 and sets the switch to its "called" position. Upon a completion of the setting, a continuous signal having frequency f.sub.3 is transmitted to the acoustic coupler 2 from the acoustic coupler 3 over the telephone line.
The acoustic couplers 2 and 3 have the same structure which is shown in detail in FIG. 2. A sound from the handset energizes a microphone 8 and is applied to a filter 6 through an amplifier 7. Thus, a received signal detection signal CD is obtained as an output from the filter 6. The signal CD is introduced into the terminal 1 or 4 to turn on the lamp L.sub.1 or L.sub.4 on the terminal. The received signal detection signal CD from the filter 6 is also applied to a modulator 11 to generate a continuous signal of frequency f.sub.1 or f.sub.3, which enters the acoustic coupler 3 or 2 at the other station over the telephone line.
As readily understood from the foregoing, the turning-on of lamps L.sub.1 and L.sub.4 at both terminals 1 and 4 indicates that the communications system is prepared for start of a communication. Therefore, after the turning-on of the lamps at both stations, a communication can be started with a so-called frequency shift signal FS of frequencies f.sub.1 and f.sub.2 at the calling party and a frequency shift signal FS of f.sub.3 and f.sub.4 at the called party.
However, in the above process required for preparing the system for start of a communication, note should be taken of the fact that up to the end of the arrangement step, transmission of all the necessary information must be carried out solely with voice. This fact means that deaf-and-dumb people can never prepare the system for start of comunication. For such people, therefore, it has been proposed to provide the acoustic couplers 2 and 3 with particular lamps L.sub.2 and L.sub.3 for indicating the state of a telephone line. In the known system for this purpose, the particular indicator lamps L.sub.2 and L.sub.3 are turned on and off in accordance with a ringing tone, busy tone, etc. In general, such communications system is designed to be portable so that a deaf-and-dumb person may carry it with him and in many cases there is used a battery as the power source for the portable system. As the indicator lamps there are usually used light emitting diodes (LEDs) or the like because of their high speed in response. However, use of such indicator lamps involves a large consumption of electric power. Therefore, the useful lifetime of the battery is shortened. This is a significant drawback of such communication system hitherto known.